In his project of semiotics, Peirce seems to have been more attentive than Saussure to the irreducibility of this becoming-unmotivated of the symbol. In his terminology, one must speak of a becoming-unmotivated of the symbol, the notion of the symbol playing here a role analogous to that of the sign which Saussure opposes precisely to the symbol:
Symbols grow. They come into being by development out of other signs, particularly from icons, or from mixed signs partaking of the nature of icons and symbols. We think only in signs. These mental signs are of mixed nature; the symbol parts of them are called concepts. If a man makes a new symbol, it is by thoughts involving concepts. So it is only out of symbols that a new symbol can grow. Omne symbolum de symbolo.
[Elements of Logic, Hartshorne and Weiss]
Peirce complies with two apparently incompatible exigencies. The mistake here would be to sacrifice one for the other. It must be recognised that the symbolic (in Peirce's sense: of “the arbitrariness of the sign”) is rooted in the non-symbolic, in an anterior and related order of signification: “Symbols grow. They come into being by development out of other signs, particularly from icons, or from mixed signs.” But these roots must not compromise the structural originality of the field of symbols, the autonomy of a domain, a production, and a play: “So it is only out of symbols that a new symbol can grow. Omne symbolum de symbolo.”